The objective of the proposed research is to further describe the behavior of structures which provide direct inputs to the final common pathways of the oculomotor system. Extracellular single unit activity will be recorded from the vestibular nuclei and mesencephalic reticular formation of alert monkeys. In order to investigate the interaction of visual and vestibular influences on discharge patterns, the monkey will be trained to perform a visual tracking task while simultaneously being oscillated about either a vertical or horizontal axis. By controlling the position of the visual target from a signal proportional to head positon, it will be possible to cause the target to move with the head or opposite to the head and at any velocity relative to the head. With this behavioral control we expect to be able to completely characterize the firing patterns of a neuron with combined visual and vestibular sensitivity. Attempts will be made to further characterize the unit synaptically by placing stimulating electrodes in the 8th nerve, the cervical spinal cord and the various oculomotor projections from functionally identified pontine reticular formation units in the cat. The role of the cerebellum in modifying the connections from the vestibular to the oculomotor nuclei will also be investigated with single unit recording in alert monkeys. Using the behavioral dissociation of vestibular and visual influences described above, unit activity in the flocculus will be related to both long and short term modification in the gain of the vestibulo-ocular reflex. Finally, single unit recordings in conjunction with lesion, stimulation and anatomical techniques will be used to study the pathway and physiology underlying the pupillary light reflex in the monkey.